How many cfms do I need?

[knight_toolworks]

I want to replace my feins and I want to get it right. each fein moves 126 cfms. so if I use two does that equal 252 cfms? or does it work that way? I mean if I can get a blower that does 156 cfms and pulls a little more vacuum the feins are about 7 or so and I am looking at a blower that does 8.9

[michael_schwartz]

Something to look at would be the curve that shows the relationship between the CFM's and vacuum. In my search for motors this has been helpful.

The suction specified for the motor is usually with the intake sealed and will usually decrease with an increase in airflow. When comparing motors I have seen all sorts of different curves.

I suspect some of the more industrial units may do a better job in this regard. But that is just a guess

[Brady Watson]

Yes - you can double CFM, but vacuum pressure will be limited to the lowest rated motor/pump. Ideally you want 14 Hg" or better and 200-300 CFM. This is what ShopBot's regen units do. If you can't run 3 phase, your options are extremely narrow...

Top of the line is a Becker VTLF series - It is absolutely the best pump on the market for this type of work. It has many advantages over a regen or roots blower, but it comes at a premium. But...It's a lot like buying an air compressor...buy the best & you'll have it til the day you die. True 'value' means different things to different people.

-B

[knight_toolworks]

yes I agree the better vac show the ratings when on full vac.
like this guy is rated a little better then a fein. more more durable of course but would it
http://www.ebay.com/itm/Republic-Regenerative-Blower-HRB400-ring-side-channel-/280504998460?pt=BI_Pumps&hash=item414f669a3c

[Brady Watson]

That's about the same specs as the Fein. If you're gonna do it...make it worth your while.

-B

[knight_toolworks]

Yes that's what I am thinking. for the money I have I can't really get more hold then what feins or fein type motors make. I do have 3 phase though.

[michael_schwartz]

Yes - you can double CFM, but vacuum pressure will be limited to the lowest rated motor/pump.

-B

So assuming the following situation.

2 motors rated to pull 136" H20 sealed off are used together, in parallel (not sucking through each other) In reality they each pull 4" H20 at their maximum airflow of 106 CFM, and lets say 80" H20 at 50 CFM.

From your explanation I understand that both motors used together would be limited to the maximum vacuum pressure of one assuming both preform the same.

However I am interested to know how this applies to a given rate of airflow when your dealing with less than the maximum vacuum, and if that stays the same or improves. For example, referring to my numbers above at 50 CFM would you still pull only 80" h20 with 2 motors rather than one, or would that number be higher.

This is where I am a bit confused.

[Brady Watson]

Yes...You would gain some 'mid-range' vacuum 'usefulness' where CFM and vacuum curves cross over each other, due to the added CFM of the 2nd vacuum. This is difficult to gauge or plot without plugging in compressor curves in MathCAD or some other statistical engineering software. Of course, there reaches a point where kerf leakage overcomes even the best of pumps, and this is really where CFM really plays an important role, aside from buffering other leaks and assisting in initial vacuum seal in order to build useful negative pressure to hold down parts. If we take things to the limit, and say we have 10 Feins, collectively pulling 7.1 Hg" and 1260 CFM, we'd have a pretty good setup able to buffer even the leakiest of situations. However, we still may not be able to hold down an entire sheet of 4x12" parts while cutting, whereas a Becker with higher negative pressure, but lower CFM would probably do it without too much trouble.

There comes a point where you don't need more CFM...and of course, there comes a point where perfect vacuum (29.92 @ sea level) cannot resist the forces of a 1/2" end mill pushing a compression bit around a 1" square. The key is to build a system that gets you in the sweet spot for CNC hold down. This seems to be anywhere in the 10-15 Hg" (working pressure) with 200+ CFM of airflow. More Hg" would be great, but CFM should be over 150.

For those that don't understand parallel/multiple vacuum principles (yeah you lurkin'), here are a few examples:

Example 1: You have 2 identical air compressors. Each makes a max of 90 psi with a max CFM of 4. Total yield out of the set of two would be: 90 psi and 8 CFM.

Example 2: You have a Fein that pulls 7 Hg" and 126 CFM and you have a Gast pump that pulls 27 Hg" and 4 CFM. Together they pull 7" Hg and around 130 CFM. The higher vacuum 'leaks' through the Fein.

Example 3: You have a Becker that pulls 25 Hg" and 175 CFM and an FPz regen vac that pulls 8.1 Hg" and 200 CFM. Together, they pull 8.1 Hg" and 375 CFM.

Now...there are ways to use a Hi/Lo setup (http://www.talkshopbot.com/forum/showthread.php?t=11813&page=2) with a valve between vacuum sources to get the best of both worlds, but it really isn't suited for leaky universal vacuum setups.

Does that help at all?

-B

[michael_schwartz]

That helps quite a bit. Thanks for the explanation.